Oil was known since Medieval times and it was normally used in combination with tempera or in fresco. This was because it took so long to dry and the drying process was speeded up a little when it was combined with other materials like tempera. Oil was normally used to retouch details of large panels that had been painted at full speed on the damp fresco plaster. The great innovation came during the [Early Flemish#ESTILOS#9] period, when investigations revolutionised the technique. Their discoveries had a lot to do with the development of alchemy, the forerunner of modern day chemistry. The advance consisted of mixing the oils not only with the mineral pigments that gave them their colour, but also with drying products that speeded up the finish. The most widely used was linseed oil, but each master and each workshop had their own secret formula that was passed orally from one generation to the next. The advantage of oil was that it allowed a painting to be done more slowly than in fresco, which had to be completed daily. It also gave the possibility of working on an uncompleted whole, instead of having to complete each section in one session. The technique allowed retouching, which meant that the composition, number of figures, colours and so on could be changed. Attention to detail and precision increased with this medium. The scene's depth also increased through an optical illusion created by the application of layers of colour, which remained opaque under layers and layers of translucent varnish that increased the painting resistance to time. Oil can be used on various supports, with hardly any change in its appearance, although there are differences in the preparation of each type of support. From the 15th century onwards the most widely used support for oil was the wood panel, developed especially by Flemish painters. Fresco is not strictly a base, but oil was used with this technique, being applied once the plaster was dry to retouch the large fragments that had been painted at full speed. Other important supports were the canvas and the metal sheet.